1. Field of the Invention
This invention is related to exploring and finding hydrocarbons in a subterranean reservoir. More specifically, this invention provides for a method for exploring and finding a subterranean hydrocarbon reservoir by modeling of temperature and/or thermal anomalies within a geologic volume of the earth's crust.
2. Description of the Prior Art
Much has been published concerning the exploration and the finding of hydrocarbons in a subterranean reservoir. Representative publications include, but far from limited to, the following: U.S. Pat. No. 4,698,759 to Eliason et al; U.S. Pat. No. 4,676,664 to Anderson et al; U.S. Pat. No. 4,120,199 to Mufti; U.S. Pat. No. 4,672,545 to Lin et al; U.S. Pat. No. 4,855,912 to Banavar et al; U.S. Pat. No. 1,901,431 to Bond; U.S. Pat. No. 3,181,629 to Birman; U.S. Pat. No. 3,808,889 to Rawson et al; U.S. Pat. No. 4,003,250 to Poppendiek et al; U.S. Pat. No. 3,344,669 to Roedder; U.S. Pat. No. 2,301,326 to Reistle, Jr.; U.S. Pat. No. 2,403,704 to Blau; U.S. Pat. No. 3,217,550 to Birman; and "Exploration Application of Temperatures Recorded on Log Headings--An Up-the-Odds Method of Hydrocarbon-Charged Porosity Prediction" by Stanton M. Ball, The American Association of Petroleum Geologists Bulletin V.66, No. 8 (August 1982), pgs. 1108-1123. All of these publications are fully incorporated herein by reference thereto.
U.S Pat. No. 4,698,759 to Eliason et al. discloses a quantitative method of geologic structural analysis of digital terrain data for implementation on a computer U.S. Pat. No. 4,676,664 to Anderson et al. discloses a method and a system for measuring the sea floor temperature gradient several meters into the formation at each of an array of measurement sites, and using these gradients to explore for and characterize hydrocarbon deposits. Mufti U.S. Pat. No. 4,120,199 proposes taking thermal gradient measurements in a borehole which does not penetrate a hydrocarbon deposit, and using them as an indication of the proximity of hydrocarbon deposits. The Mufti patent suggests that it may be practical in some cases to drill a number of shallow test holes in an area to be explored simply for the purpose of mapping temperature gradients from measurements taken in the 50 to 100 foot depth region and that even shallower boreholes can be used when a set of readings can be made in such a short time period that seasonal changes can be ignored. The Lin et al. U.S. Pat. No. 4,672,545 teaches converting seismic data obtained at known points to synthesized seismic traces obtainable at arbitrarily selected points. Two dimensional seismic data in the Lin et al. patent are converted to three dimensional data with the aid of a programmed computer to permit generation of arbitrary views of a particular geologic structure as well as a mathematical representation of the structure. U.S. Pat. No. 4,855,912 to Banavar et al. discloses investigating earth formations surrounding a borehole by selectively heating a region of the formations, and measuring the thermal response to obtain useful information concerning the characteristic length scales of the pores in the heated region. The thermal response is obtained indirectly by measuring the electrical conductivity response. U.S. Pat. No. 1,901,431 to Bond discloses a method and apparatus for accurately determining rock temperatures in a well. U.S. Pat. No. 3,181,629 to Birman teaches the use of subsurface temperature measurements in earth prospecting. Rawson et al. in U.S. Pat. No. 3,808,889 discloses a heat flow transducer for use in measuring the geothermal heat flow in the earth. Likewise U.S. Pat. No. 4,003,250 to Poppendiek et al. also discloses a heat flow transducer. U.S. Pat. No. 3,344,669 to Roedder relates to heat sensing instruments which are operative to indicate and measure relevant characteristics of thermal gradients, and heat sensitive control devices adapted to function in response to variations in such characteristics of thermal gradient. U.S. Pat. No. 2,301,326 to Reistle, Jr. is directed to a method for determining the temperature at a series of points, or the temperature gradient, of a flowing well, and from the results obtained determining the position in the bore hole at which the oil components and gaseous components produced by the well enter the bore hole. U.S Pat. No. 2,403,704 to Blau discloses a method for prospecting for oil by determination of subsurface thermal properties. The Birman U.S. Pat. No. 3,217,550 relates to methods of geophysical prospecting which permit the detection and location beneath the earth's surface of a body of mineral, fluid, or rock mass possessed of anomalous thermal characteristics as well the detection of depth variations of such body from point to point over a given surface ore. Stanton M. Ball in "Exploration Application of Temperatures Recorded on Log Headings--An Up-the-Odds Method of Hydrocarbon-Charged Porosity Production" teaches an analytical technique for defining temperature anomalies called "hotspots" which are hydrocarbon temperature anomalies. Ball more specifically teaches that hydrocarbon fluids insulate more and their temperatures are elevated more easily than contiguous waters. Hydrocarbon reservoirs whose heat flow effects are not obscured by the anisotropic effects of adjacent water reservoirs, may cause definable temperature anomalies. The technique formulated by Ball more specifically involves the following steps: (1) calculation of geothermal gradient values, (2) creation of a geothermal gradient field areally, (3) vector analysis or contouring of created data, and (4) temperature anomaly definition.
Thus, temperature anomalies in connection with hydrocarbon reservoirs have long been recognized. However, the complexities of heat flow under the surface of the earth and the resulting temperature distribution have prevented widespread success in utilizing thermal data to locate hydrocarbon accumulations. Even the most elementary conclusions as to what features of thermal data should be examined in order to get an indication of associated hydrocarbons were widely debated with no generally acceptable conclusions possible Some authors have espoused the view that hot temperature anomalies occur over hydrocarbon reservoirs. Other authors, such as Fons in U.S. Pat. No. 4,476,716 incorporated herein by reference thereto, discloses that hydrocarbons accumulations can be located by the presence of a cool temperature anomaly above the hydrocarbon accumulation(s). The Fon's method seeks cool anomalies over a hydrocarbon accumulation and defines an anomaly as any observed temperature which differs significantly from the average observed temperature behavior in the immediate vicinity of the anomaly. This technique has proven statistically to be a reliable method of locating hydrocarbon reservoirs on the Texas Gulf coast, but in other provinces, the technique has been less successful than desired.
There are many factors that influence and cause temperature anomalies in the crust of the earth. Meyer, H. J. et al., in "The Relationship of Geothermal Anomalies To Oil And Gas Accumulation in The Rocky Mountain Area", Am. Assn. Petl. Geol. Bulletin, 1983 concludes that upward fluid movement at depth was an important factor. Other factors include, but are not limited to, the following: variation in heat flow within the earth; reduced thermal conductivity of hydrocarbon bearing reservoir rock as compared to similar rock bearing only formation water; and variation in thermal conductivity within the crust of the earth separate or apart from those variations caused by hydrocarbon reservoirs.
Of the factors, the reduced thermal conductivity of hydrocarbon bearing reservoir rock as compared to similar rock bearing only formation water is of the highest interest. It is known that the heat or thermal conductivity of a material depends upon the conductivity of the solid particles, the geometry of the solid particles, and the conductivity of any fluids comprised between the solid particles. The heat conductivity of quartz is greater than that of calcite, while the latter is greater than that of the usual silicates. In heterogeneous substances the heat conductivity increases with compactness. For instance, igneous and metamorphic rocks are usually better conductors than sediments. The nature of the fluids present in the pores is also an important factor because if the material is dry and very porous, the heat conductivity is considerably less than if the pores contain water (sediments). This is due to the fact that air is a much poorer heat conductor than water. Oil and natural gas are also poor conductors; therefore, the heat conductivity of petroleum-bearing reservoirs should be smaller than that of the same reservoirs when they contain only water. The reduced thermal or heat conductivity of hydrocarbon bearing reservoir rock as compared to similar rock bearing only air and/or formation water relates directly to the presence or absence of hydrocarbons within the pores of the rock. If the effects of hydrocarbon accumulations in hydrocarbon bearing reservoir rock could be modeled, and those effects isolated from the various other factors influencing temperature or thermal anomalies by the employment of the model, then the location of hydrocarbon accumulations of hydrocarbon accumulations by their influence upon the observed temperatures and/or heat flow emanating from the crust of the earth becomes practical.
Much effort has been extended into modeling, including the modeling geological structures as an aid to exploring and finding hydrocarbons and other subterranean substances. U.S. Pat. No. 4,821,164 incorporated herein by reference thereto, to Swanson discloses modeling of geologic volumes of the earth's crust. The patent more particularly discloses a system for developing a model of a geologic volume by locating positions of observations within the model which corresponds to known positions of observation in the geologic volume, and thereafter extrapolating from or interpolating between such positions of observations. The model is composed of a plurality of small incremental volumetric elements configured to resemble corresponding incremental volumetric elements in the geologic volume. Swanson further discloses that modeling may be performed in several ways, as for example, by making maps or sections of volumes directly from the information. Generally speaking, a map is a two-dimensional projection on a horizontal planar surface of a representation of features of the volume modeled. A section, on the other hand, is normally a graphic representation of the volume projected on a vertical plane cutting the volume. Another way to model as disclosed by Swanson is to systematically store the information in computers, and thereafter recover the information as desired. Recovery of the stored information in some instances may involve feeding the information to plotters which automatically plot the data in map or section form. In general, then, the art of modeling a geologic volume in a first aspect resides in building a model of the volume by assembling known data as well as extrapolated and interpolated data throughout the modeled volume. Once the model is built, displays such as maps, cross-sections, and statistical information result from the model. Modeling the earth's crust, including map and section making, involves complex geological and geophysical relationships and many types of data and observations. Of particular interest in the present invention are geological volumes of sedimentary rocks or deposits since almost all oil and gas, many mineral deposits, and most ground water normally occur in sedimentary deposits--typically in porous reservoirs such as clastic (sandstones), secreted, and/or precipitated deposits.
There has been considerable effort extended in the oil and gas industry towards modeling of fluid or liquid flow in porous media by attempting to relate fluid or liquid flow to pressure gradients and rock properties. There are many analogies between the flow of a fluid or liquid through a permeable medium, the flow of electricity through a conductor and the flow of heat by conduction (as opposed to convection and radiation) through a solid body. In particular, the mathematical solutions of the problems involved in these three branches of physics are identical and many formulas obtained for one of them can be used for the others by simply translating each symbol into its proper analogue. The fundamental law of heat conduction is Fourier's law (i.e. dQ/dt=kA dT/dx where Q is amount of heat flowing in differential time dt, k is proportionality factor known as heat or thermal conductivity of the medium, A is path cross section, and dT/dx is the rate of change of temperature, T, with respect to the length of path, x). Fourier's law is analogous to D'Arcy's law in hydrodynamics and to Ohm's law in electricity. D'Arcy's law is dQ/dt=kA dP/dx where dQ/dt is fluid output or input per unit time, k is permeability, A is path cross section, and dP/dx is the rate of change of pressure, P, with respect to the path length, x. Ohm's law is dQ/dt=CA dV/dx where dQ/dt is current intensity or change in current per unit time, C is electrical conductivity, A is path cross section, and dV/dx is the rate of change of potential or voltage, V, with respect to path length, x.
The flow of heat within the crust of the earth obeys a partial differential equation or diffusivity equation that is identical in form to the equation for fluid flow. For example, the partial differential equation or diffusivity equation in radial form for fluid flow is: ##EQU1## where p is pressure, .phi. is porosity, k is permeability, x and y and z are respectively length in x, y, and z direction, t is time and C.sub.t is system compressibility. For incompressible fluid flow C.sub.t becomes O so the equation reduces to: ##EQU2## For pseudo steady-state compressible liquid flow in bounded systems, .differential.p/.differential.t is a constant, i.e., the pressure is falling everywhere at the same rate A, and ##EQU3##
The diffusivity equation for the flow of heat is ##EQU4## while .theta. (x, y, z,t) represents the temperature at any point in space at time t, and it is assumed that heat flows in the direction of decreasing temperature and that the rate (in calories per second) across any infinitesimal square body of mass or matter is proportioned to the area of the square body of mass or matter; and k is a constant in calories per centimeter per degree per second, .rho. is the density of the mass or matter in grams per cubic centimeter and c is the specific heat in calories per gram per degree. For steady-state flow of heat becomes O and the equation reduces to: ##EQU5##
Many different techniques have been developed which permit solving the basic diffusivity equation on a computer for three dimensional and time varying flow. One method known as line successive over relaxation (LSOR) is extremely reliable but normally requires many, many computations in order to successfully converge to reach the correct desired solution. Another technique known as Newton-Rapson, generalized iterative technique for solving for the roots of algebraic equations, greatly increases the speed for convergence. For fluids and heat flow, most flow is in one direction or in one plane, such as in the horizontal direction or horizontal plane which involves in general, two coordinates. If vertical flow is also present, the problem then is three dimensional and the effort required to reach convergence on a computer is correspondingly increased. If LSOR is employed in all three dimensions, the computer time (and cost) required is extremely high.
In view of the known prior art for modeling subterranean hydrocarbon reservoirs and for solving the diffusivity equation on a computer, it is believed that a need exists to find a way to explore and find a subterranean hydrocarbon reservoir by modeling of temperature and/or thermal anomalies within a geologic volume of the earth's crust which would include a cost effective and accurate method for solving a heat flow diffusivity equation.
Important aspects of the present invention are directed to meeting these needs.